When a dermal cell of a mouse is transformed in vivo by ultraviolet it forms a large clone of cells which appears in the form of a skin tumor. The cell surfaces of most tumors of this type display antigenic molecules which stimulate an immune response and tumor rejetion even in the same highly inbred strain. Even more remarkable is the finding that each tumor line displays antigenic molecules differing from all other cell lines so far tested. These molecules are a stable and heritable property of each of the tumor lines. Our research program is aimed at characterizing these antigenic molecules using immunological techniques and high resolution methods of molecular analysis. We wish to know how they relate to each other and to normal cell surface molecules found in embryos or adults. Are they abnormal molecules somehow generated as the cells are transformed into tumors or might they be normal gene products such as those found on cell surfaces in embryos? It seems likely that the insights gained will be relevant to both the cancer problem and fundamental questions relating to the genetic control of cell surface specificity and diversity.